412 Mr. F. Jenkia on the Retardation of 



One good example of the kind of instrument alluded to is 

 afforded by Professor Wheatstone's automatic arrangement, with 

 which, on a single circuit, 600 letters per minute can be transmit- 

 ted, each indicated by about three dots on an average, requiring, 

 when reverse currents are used, that they shall be transmitted 

 at the rate of 3600 per minute, or 60 per second. 



Other automatic arrangements, for instance that of the Che- 

 valier Bonelli, differing exceedingly in most points from that 

 above mentioned, have this in common, that the number of dis- 

 tinct currents required per minute is exceedingly great; and 

 when these arrangements are used on long aerial circuits, the 

 phenomena of induction place a limit to the number of messages 

 that can be sent in a given time, precisely as the induction in 

 submarine cables limits the capability of the line with ordinary 

 hand-signalling. It becomes, therefore, of importance to the 

 inventors and introducers of these inventions to know where to 

 expect this limit to the rate of transmission, or to the distance 

 over which messages can be sent at a given rate. This consi- 

 deration led the writer to examine whether any existing experi- 

 ments gave the data required ; and he found that a paper by 

 M. Guillemin, published in I860*, afforded, for the particular 

 lines of which he made trial, much of the information wanted. 

 M. Guillemin himself appears to have imperfectly understood 

 the theory of Professor W. Thomson ; but it is somewhat difficult 

 to be certain as to his views on this point, owing to a very curious 

 coincidence. Ohm, in his paper " On the Mathematical Law of 

 the Galvanic Circuit," published in 1827f, not only considers the 

 case of the permanent current which exists according to the well- 

 known relation between electromotive force and resistance known 

 par excellence as Ohm's Law, but also considers the condition of 

 the current when increasing from nothing to its maximum. 

 Ohm does not suppose that this change can take place instanta- 

 neously, and upon purely hypothetical grounds he arrived at 

 an expression of the law according to which the increase must 

 take place. This law was wholly unsupported by any practical 

 experiment, and appears at the time to have attracted little or 

 no attention. It is, however, exactly that which was later 

 discovered by Professor William Thomson in a more complete 

 form, and with a knowledge of experimental results justifying 

 his conclusions. Ohm's expression is 



du _ dht 



rf Tt^ x dx^ i 



where u is the potential at the time t, and at a point at the 



* Annates de Chimie et de Physique, 3 ser. vol. lx. 

 f Vide Taylor's Scientific Memoirs, vol. ii. 



